The use of magnetic sensors in cooperation with, for example, one or more projections on a shaft to give an output from which shaft rotational speed or torque may be determined is well known. In such sensors, a voltage induced in a coil by changes in a magnetic flux pattern of a magnetically energised pole piece, caused by a movement of a body of magnetic material in the field of the pole piece is detected and measured.
The rotational speed of a rotating element can be determined by measuring the frequency of the periodic signal produced in the coil as one or more markers or elements on the surface of the rotating element pass through the field of the pole piece.
EP-A-169670 and U.S. Pat. No. 4,739,260 describe transformer probes for detecting movement of magnetic objects in which one end of a closed loop electrical circuit formed from an electrical conductor is wound around a pole piece such that movement of a magnetic body through a first flux pattern generated by the pole piece induces a current in the conductor, which generates a second flux pattern at the second end of the closed loop, remote from the first end, which forms a primary coil of transformer. The primary coil is inductively coupled to a multi-turn secondary coil terminated at an output connected to a detecting apparatus. When the resistance of the closed loop electrical circuit is low, a magnetic object passing in the vicinity of the pole piece induces a low voltage, high current signal in the primary coil, which is transformed into a high output voltage at the output of the secondary coil.
GB 2387656 discloses a transformer probe in which a shorting turn of a conductor is inductively coupled to the primary and secondary coils of a transformer probe to limit the output voltage at high speeds. The shorting turn acts to limit the output voltage at the output of the secondary coil at high frequencies, i.e. when the rotational speed of the magnetic object being detected is high.
Multiple channel transformer probes provide multiple output signals from a single speed sensor. One application of this type of probe is as a speed sensor in aircraft engine control systems. Speed sensors in aircraft control systems are provided with multiple output channels for reasons of redundancy and system safety. The multiple channels can be provided by coupling the primary circuit to multiple secondary coils, each wound around a separate transformer core. However, as the multiple outputs generated by the multiple secondary coils are to provide redundancy, it is very important that a fault in one output channel does not adversely affect the output in another output channel. This can occur when multiple output channels are coupled to the same primary circuit.
This problem is particularly acute at low speeds, because at low speeds the output voltage from each channel is relatively low. It is an aim of the present invention to provide a system to ensure sufficient output voltage at low speeds, even when one or more output channels have an open circuit fault.